1. Field of the Invention
The present invention relates generally to downhole tools, such as bridge and frac plugs, used in oil and gas wells.
2. Related Art
Oil and gas wells are completed using a complex process involving explosive charges and high pressure fluids. Once drilling is complete, a well is lined with steel pipe backed with cement that bridges the gap between the pipe outer diameter (OD) and rock face. The steel/cement barrier is then perforated with explosive shaped charges. High pressure fluids and proppants (spherical sand or synthetic ceramic beads) are then pumped down the well, through the perforations and into the rock formation to prepare the rock for the flow of gas and oil into the casing and up the well. This fracturing process is repeated as many times as needed.
Another technological improvement has been the use of composite plugs used to complete these unconventional wells. Oil and gas wells are completed using a complex process whereby steel casing pipe is secured in place with cement. The steel/cement barrier and surrounding oil and gas bearing rock layers are then perforated with shaped charges in order to start the flow of oil and gas into the casing and up to the wellhead. As they prepare to perforate at each level, well technicians set a temporary plug in the bore of the steel casing pipe just below where they will perforate. This plug allows them to pump “Frac fluids” and sand down to the perforations and into the reservoir. This fractures the rock and props open the fractures allowing the movement of gas or oil towards the well at that level. Use of the temporary plug prevents contamination of already-fractured levels below. This process is repeated up the well until all desired zones have been stimulated. At each level, the temporary plugs are left in place, so that they can all be drilled out at the end of the process, in a single (but often time-consuming) operation. The ability to drill all the temporary composite plugs in a single pass (often taking only one day) compared to taking days or weeks to drill cast iron plugs has radically changed well completion economics.
One problem encountered during drilling is that as the upper end of the plug is milled away the plug assembly loses its grip on the casing and the lower part of the plug body drops (in a vertical well) down to a lower level. In a horizontal well the lower plug body falls to the low side of the casing. In both vertical and horizontal wells there is often 10 to 100 feet of loose sand that as accumulated on top of the next plug. This sand is from the fracing operation performed at that level. In order to drill out the next plug, the drill bit or mill has to remove the sand column above the plug. As the drill bit or mill is moved to the next station it continues to turn. The mill catches the lower plug body and begins to spin it.
Permanent and temporary plugs use various designs at their upper and lower ends that are intended to allow the lower plug body to lock up (i.e. prevent rotation) to the top of the next plug to improve drill out rates. For example, angled top and bottom ends assure that the plug remnant from an upper stage will engage the top end of the lower plug and not spin when being drilled out. As another example, other plugs accomplish “lock up” with a half circle style of cut at each end. As another example, other plugs have a crenellated lower end and an internal thread in the lower end that matches a thread at the upper end so it appears that lock up occurs when the mill spins the lower end onto the thread on the upper end.
Examples of downhole tools include U.S. Pat. Nos. 5,540,279 and 6,491,108.